Problem: In 21 years, Nadia will be 4 times as old as she is right now. How old is she right now?
Explanation: We can use the given information to write down an equation about Nadia's age. Let Nadia's age be $n$ In 21 years, she will be $n + 21$ years old. At that time, she will also be $4 n$ years old. Writing this information as an equation, we get: $n + 21 = 4 n$ Solving for $n$ , we get: $3 n = 21$ $n = 7$.